1. Field of the Invention
The invention relates to a display device such as an electronic paper, an electronic paper file and the like, which comprising a sheet-like display unit provided with data-writing means and data-eliminating means, for example.
2. Description of the Related Art
The more the recent information technology society progresses, the more the technology for a display and a hardcopy get to be important. The paper, which has always been used as a medium of information communication, is very usable still now regarding the portability, the record-ability and the method of recognizing information by human. Therefore, not only a display medium such as CRT and LCD, but also a sheet-like display medium including the memory function has been developed recently.
As the above sheet-like display medium, the concept of an electronic paper is proposed (in Japanese laid-open publication No. 10-171620), and an electronic paper file is also proposed in which a plurality of electronic papers are bound into a book (PCT Japanese translation publication No. 11-502950). In addition, as a recording material to be applied to the display medium various rewritable recording materials are studied (See pp 209 to 251 of “Japan Hardcopy'99—Theses” published by The Imaging Society in Japan).
One of the rewritable recoding mediums is a nonvolatile material, which is applied to the electronic paper. There is a display medium using the smectic liquid crystal of the transmissive liquid crystal and the Guest-Host type of the liquid crystal/polymer composite film adopting the dichronic dye as the nonvolatile materials, for example. In the display medium, the dichronic dye is oriented together with the liquid crystal by heat and gets to be in the focal conic state; thereby the light absorption increases, and the display medium gets luminous. Additionally, by loading the electric charge on the display medium, the focal conic state changes to the homeotropic state, and then the colors fade away.
A display medium 30 as shown in FIG. 10 is configured so as a front-light 31 is provided to under a display unit 34 intervening an air-layer 35 between them; the display unit 34 using a reflective liquid crystal as the nonvolatile material. The front-light 31 comprises a light guide plate 33 and a light source 32 such as a cold-cathode tube or LED attached to one side of the light guide plate 33. Moreover, either of the upper surface or the lower surface of the light guide plate 33 is formed in a shape of prism consecutively so that the light from the light source 32 may irradiate a whole surface of the display unit 34.
The liquid crystal used by the display unit of the above-mentioned display medium is the nematic type or the smectic type. In the either case, when the display is performed in the color mode of Red (R), Green (G), and Blue (B), three liquid crystals corresponding to the RGB must be arranged in parallel on a plane surface in order to display one pixel. That is to say, the number of pixels on the display unit in the color mode requires three times as much as that in the monochrome mode.
As shown in the configuration of the conventional display medium 30, when there is the air-layer 35 between the display unit 34 using the reflective crystal liquid and the light guide plate 33 of the front-light 31, a light coming in at a specific incident angle, which is one of the lights coming into the display unit 34 from the light guide plate 33, reflects on the surface of the display unit 34, therefore the contrast and the coefficient of utilization of light decrease.